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JOURNAL OF COLLOID AND INTERFACE SCIENCE 176, 286–292 (1995) Fluorescence of ZnO Ultrafine Particles Quenched by Naphthothiacarbocyanine Dye in Ethanol: The Effect of Water SIHAI CHEN,* , ²U LRICH NICKEL,* ,1 AND XINMIN REN² * Institut fu ¨r Physikalische Chemie, Universita ¨t Erlangen-Nu ¨rnberg, Egerlandstrasse 3, 91058 Erlangen, Germany; and ² Institute of Photographic Chemistry, Academia Sinica, Beijing 100101, P.R. China Received January 13, 1994; accepted March 9, 1995 in ethanol by Koch et al. in 1985 (12), the photophysical The interaction between ZnO ultrafine particles and naphthothi- and photochemical properties of colloidal ZnO have been acarbocyanine ( NTC ) dye in ethanol was investigated by monitor- intensively studied (13–18). Two fluorescence bands are ing the variations in intensity of visible fluorescence of ZnO with normally observed for ZnO. One is the excitonic fluores- increasing concentration of NTC at different temperatures. It was cence, which is caused by the direct combination of pho- found that the presence of small amounts of water in the solution toexited electrons and holes. The other is the visible fluores- greatly affects the aggregation state of NTC and thus the fluores- cence, which is the radiation that results from the relaxation cent behavior of ZnO. In the absence of water, a dynamic quench- of the excited electrons on the sites of trapped holes on the ing was observed by NTC dye when its concentration was less surface. The trapping of the photoinduced electron on the than 7 mM and 13 mM at 207C and 427C, respectively. But at higher concentrations, the severe aggregation of NTC resulted in surface of particle has also been found to be very fast and a great decrease of NTC dye concentration in solution, leading to the average diffusion time from bulk to surface has been a decreased quenching ability of NTC. When the quencher was reported to be less than 0.2 ps for a particle of 4 nm diameter dissolved in 95% ethanol (in the mixed ZnO–NTC solution, final (11). This implies that the visible fluorescence of ultrafine water content 0–0.5%), aggregation of NTC did not occur. How- particles is a reflection of the properties of the particle sur- ever, if the quencher was dissolved in water (final water content face. Therefore, it is possible to investigate the interfacial in mixed ZnO–NTC solution 1.5%–7%), due to the adsorption interaction by utilizing visible fluorescence measurement. of water by ZnO, coagulation of ZnO particles occurred first, then Cyanine dyes, which are widely used as spectral sensitiz- a new aggregation state of NTC-containing water was formed on ers in photography (19), show some interesting behaviors the ZnO surface, leading to a stronger fluorescence quenching. in the photoinduced charge and energy transfer in the silver q 1995 Academic Press, Inc. halide microcrystalline system. The most fascinating aspect is their unique aggregation states on different substrate sur- faces. Attempts have been made to sensitize semiconductors with cyanine dyes utilizing this property (20, 21). In the INTRODUCTION present report, the interfacial interaction between ZnO ul- trafine particles and NTC dye in ethanol was studied. Differ- The photophysics, photochemistry, and photoelectro- ent interfacial interactions were observed when the aggrega- chemistry of ultrafine semiconductor particles are areas of tion state of NTC dye was altered by the introduction of active research in recent years because of their potential water. applications in material science, industrial processes, and environmental science (1–9). Of particular interests is the EXPERIMENTAL sensitization of large band gap semiconductor particles, such as ZnO or TiO 2 , with sensitizers or some suitable redox Materials couples. The improved efficiency of photoinduced charge 3,3 * - di - ( 3 - sulfopropyl ) - 4,5,4 *,5 * - dibenzothiacarbo- separation for these large band gap semiconductor particles cyanine hydroxide, triethylamine salt ( NTC ) was synthe- by coating with dye or coupling with narrow band gap semi- sized by Dr. Wenpeng Yan in the Institute of Photographic conductor particles has been reported (10, 11). Chemistry. The structure and purity was ascertained by ele- Since the successful preparation of ZnO ultrafine particle mental analysis, NMR, and IR spectrometry. All reagents used were of analytical grade. Bidistilled water was applied 1 To whom correspondence should be addressed. throughout. 286 0021-9797/95 $12.00 Copyright q 1995 by Academic Press, Inc. All rights of reproduction in any form reserved.

Fluorescence of ZnO Ultrafine Particles Quenched by Naphthothiacarbocyanine Dye in Ethanol: The Effect of Water

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JOURNAL OF COLLOID AND INTERFACE SCIENCE 176, 286–292 (1995)

Fluorescence of ZnO Ultrafine Particles Quenchedby Naphthothiacarbocyanine Dye in Ethanol:

The Effect of Water

SIHAI CHEN,* ,† ULRICH NICKEL,* ,1AND XINMIN REN†

*Institut fur Physikalische Chemie, Universitat Erlangen-Nurnberg, Egerlandstrasse 3, 91058 Erlangen, Germany;and †Institute of Photographic Chemistry, Academia Sinica, Beijing 100101, P.R. China

Received January 13, 1994; accepted March 9, 1995

in ethanol by Koch et al. in 1985 (12), the photophysicalThe interaction between ZnO ultrafine particles and naphthothi- and photochemical properties of colloidal ZnO have been

acarbocyanine (NTC) dye in ethanol was investigated by monitor- intensively studied (13–18). Two fluorescence bands areing the variations in intensity of visible fluorescence of ZnO with normally observed for ZnO. One is the excitonic fluores-increasing concentration of NTC at different temperatures. It was

cence, which is caused by the direct combination of pho-found that the presence of small amounts of water in the solutiontoexited electrons and holes. The other is the visible fluores-greatly affects the aggregation state of NTC and thus the fluores-cence, which is the radiation that results from the relaxationcent behavior of ZnO. In the absence of water, a dynamic quench-of the excited electrons on the sites of trapped holes on theing was observed by NTC dye when its concentration was lesssurface. The trapping of the photoinduced electron on thethan 7 mM and 13 mM at 207C and 427C, respectively. But at

higher concentrations, the severe aggregation of NTC resulted in surface of particle has also been found to be very fast anda great decrease of NTC dye concentration in solution, leading to the average diffusion time from bulk to surface has beena decreased quenching ability of NTC. When the quencher was reported to be less than 0.2 ps for a particle of 4 nm diameterdissolved in 95% ethanol ( in the mixed ZnO–NTC solution, final (11). This implies that the visible fluorescence of ultrafinewater content 0–0.5%), aggregation of NTC did not occur. How- particles is a reflection of the properties of the particle sur-ever, if the quencher was dissolved in water (final water content face. Therefore, it is possible to investigate the interfacialin mixed ZnO–NTC solution 1.5%–7%), due to the adsorption

interaction by utilizing visible fluorescence measurement.of water by ZnO, coagulation of ZnO particles occurred first, thenCyanine dyes, which are widely used as spectral sensitiz-a new aggregation state of NTC-containing water was formed on

ers in photography (19), show some interesting behaviorsthe ZnO surface, leading to a stronger fluorescence quenching.in the photoinduced charge and energy transfer in the silverq 1995 Academic Press, Inc.

halide microcrystalline system. The most fascinating aspectis their unique aggregation states on different substrate sur-faces. Attempts have been made to sensitize semiconductorswith cyanine dyes utilizing this property (20, 21). In theINTRODUCTIONpresent report, the interfacial interaction between ZnO ul-trafine particles and NTC dye in ethanol was studied. Differ-The photophysics, photochemistry, and photoelectro-ent interfacial interactions were observed when the aggrega-chemistry of ultrafine semiconductor particles are areas oftion state of NTC dye was altered by the introduction ofactive research in recent years because of their potentialwater.applications in material science, industrial processes, and

environmental science (1–9). Of particular interests is the EXPERIMENTALsensitization of large band gap semiconductor particles, suchas ZnO or TiO2, with sensitizers or some suitable redox Materialscouples. The improved efficiency of photoinduced charge

3,3* - di - (3 - sulfopropyl) - 4,5,4*,5 * - dibenzothiacarbo-separation for these large band gap semiconductor particles

cyanine hydroxide, triethylamine salt (NTC) was synthe-by coating with dye or coupling with narrow band gap semi-

sized by Dr. Wenpeng Yan in the Institute of Photographicconductor particles has been reported (10, 11).

Chemistry. The structure and purity was ascertained by ele-Since the successful preparation of ZnO ultrafine particle

mental analysis, NMR, and IR spectrometry. All reagentsused were of analytical grade. Bidistilled water was applied

1 To whom correspondence should be addressed. throughout.

2860021-9797/95 $12.00Copyright q 1995 by Academic Press, Inc.All rights of reproduction in any form reserved.

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287QUENCHING OF ZnO BY DYE: WATER EFFECT

Preparation of NTC quenchers. The quencher, 250 mMNTC, was dissolved in three different solvents: (A) pureethanol (denoted as NTC(A)), (B) 95% ethanol (denotedas NTC(B)), and (C) in water (denoted as NTC(C)). Solu-tions A and C showed purple and red-purple colors, respec-tively.

Preparation of ZnO colloids. Ten milliliters of 0.02 MNaOH was diluted with 60 ml ethanol. Two milliliters of0.04 M Zn(CH3COO)2 in ethanol was then added undervigorous stirring. After 40 min, 26 ml ethanol and 2 ml ofthe Zn(CH3COO)2 solution was introduced. A transparent1 1 1003 M ZnO sol with 6 1 1004 M Zn2/ in excess (pHÅ 6) was obtained after 24 h of stirring. The colloids wereused after aging for about two weeks.

Methods

Fluorescence spectra were recorded with a Hitachi MPF 4spectrofluorometer equipped with a thermostated cell holder.Fluorescence was detected in a right angle viewing mode.Excitation and emission slits were set at 4- and 6-nm band-pass. The excitation wavelengths for the ZnO sol and NTCdye were 310 nm and 580 nm, respectively. The absorption

FIG. 1. (A) Fluorescence spectra of 1 1 1003 M ZnO quenched byspectra were obtained with the aid of a Hitachi 330 and aNTC(A). (B) Fluorescence spectra of NTC(A) in the absence of ZnO.Hitachi 557 spectrophotometer.Concentrations of NTC(A) increasing in the direction of the arrow are:The electron images of ZnO were observed with a Philips0, 1.24, 3.69, 6.10, 8.45, 10.77, 13.03, 15.26, and 17.44 mM . Excitation

EM 400 transmission electron microscope. The mean diame- wavelength is 310 nm. Temperature is 207C.ter of the ZnO particles was around 3 nm.

F Å Ar f1r f2 , [1]RESULTS AND DISCUSSION

where A is the absorbance of ZnO sol at the excitation wave-Figure 1A shows the visible fluorescence spectra of thelength. The variable f1 represents the trapping efficiency ofZnO sol with increasing addition of NTC(A). A new fluo-anion vacancies for the photogenerated electrons. The vari-rescence band centered at 620 nm appeared for greaterable f2 is the emission efficiency of the trapped electrons.NTC(A) concentrations. Compared with the fluorescenceWhen NTC is present, due to the absorption (A*) of NTCspectra for NTC solution in the absence of ZnO (Fig. 1B),at the excitation wavelength (310 nm) and its effect on theobviously, this new peak is the fluorescence of NTC dye.fluorescence of ZnO, the emission intensity of ZnO (F *)The dye also shows absorbance at the excitation wavelengthchanged toof 310 nm. Although the absorption of NTC dye at 310

nm would decrease the whole amount of photons that ZnOadsorbed, this effect can be neglected due to the much F * Å (A 0 A *)r f *1r f *2 . [2]smaller absorbance of NTC at 310 nm relative to that ofZnO. More details are explained below.

From [1] and [2],As shown in Fig. 1A, ZnO sol in the absence of NTCexhibits a fluorescence band with its lmax around 520 nm.The origin of this emission has been discussed by several F * /F Å (A 0 A *) /Ar f *1 / f1r f *2 / f2 . [3]authors (11–13). Koch et al. attributed it to anion vacancies(12). This interpretation was later questioned by Bahnemann Because the absorbance of NTC at 310 nm under ouret al. (13), who suggested that it is caused by the photogen- experimental conditions is much smaller than that of ZnOerated, trapped electrons tunnelling to preexisting, trapped sol, the first term in [3] is approximately unity. Thus,holes. Based on our recent experimental results, we agreewith the viewpoint of Koch et al. (22). The intensity ofvisible emission of ZnO, F , can be expressed as F * /F à f *1 / f1r f *2 / f2 . [4]

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288 CHEN, NICKEL, AND REN

FIG. 2. Fluorescence quenching of ZnO colloid by NTC at different temperatures expressed in the form of Stern–Volmer plots. I0 and I are thefluorescence intensities of ZnO colloid in the absence and presence of NTC, respectively. The quencher of NTC was dissolved in (A) absolute ethanol,(B) 95% ethanol, and (C) water.

The fluorescence of ZnO is mainly affected by the surface dissolved in different solvents. For a variety of conditions,interaction with NTC. The emission intensity of ZnO after plotting I0 /I vs the concentration of quencher, Q , yieldsthe addition of NTC can be obtained by simply excluding straight lines with gradients denoted conventionally as Kq t,the fluorescence of NTC from the whole fluorescence area.

Figure 2 shows the Stern–Volmer plots for the visiblefluorescence of ZnO sol quenched by NTC stock solution I0 /I Å 1 / Kq t [Q] . [5]

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289QUENCHING OF ZnO BY DYE: WATER EFFECT

FIG. 2—Continued

Bahnemann et al. reported that 80% of intensity decays spectrum. According to the discussion of West et al. (23,24), the bathochromic shift of the monomeric absorptionwithin the characteristic time (t) of 17 ns for 1003 M suspen-

sion of ZnO sol in 2-propanol (13). A value of 10 ns was maximum is related to the substrate’s refractive index (23).It is termed the Ma band and is caused by the dye moleculesgiven by Koch et al. for aqueous ZnO colloid (12). Kamat

et al. used a three-component decay law to fit the decaycurve, the first two parts of the lifetime for fluorescence at

TABLE 1520 nm were 2.42 ns and 17.02 ns, respectively (11). In ourQuenching of Visible Emission of ZnO Colloid by NTC undercase, the bimolecular quenching constants were calculated

Different Conditionsassuming a t value of 17 ns. The results are summarized inTable 1.

Quencher Kqt/M01(1104) Kq/M01 S01(11012)a

In the case without water (Fig. 2A), the Stern–Volmerplots are linear and the gradients increase with temperatures, NTC(A)

37C 5.4 3.18it is easy to conclude from this that at lower NTC concentra-207C (õ7 mM) 7.2 4.23tions, a dynamic interaction between ZnO and NTC oc-207C (ú7 mM) 5.2 3.05curred. Interestingly, two inflection points were found in the427C (õ13 mM) 8.0 4.71

Stern–Volmer plots, at 7 mM and 13 mM at 207C and 427C, 427C (ú13 mM) 3.8 2.21respectively. In each case, the Stern–Volmer constant ( i.e.,

NTC(B)b

the gradient) decreased at higher NTC concentrations (Table37C 4.0 2.35

1) , showing the sudden decrease of the quenching ability 207C 4.8 2.82of NTC. 427C 5.9 3.49

Why does the quenching ability of NTC decrease at higherNTC(C)b

concentrations. This can be explained by the change in the 37C (õ7 mM) 3.7 2.20aggregation state of NTC dye. The absorption spectra of 207C (õ7 mM) 6.3 3.68

427C (õ7 mM) 7.5 4.40monomeric NTC dye exhibited a peak at 602 nm and a smallshoulder on the shorter wavelength side (similar to curve c

a Assuming t Å 17 1 1009 s for the visible fluorescence of ZnO colloid.in Fig. 3) . After the concentrated NTC was mixed with ZnOb Not only water molecule change the aggregation state of NTC, it also

sol (final concentration 27.8 mM NTC and 8.8 1 1004 M adsorbs on the surface of ZnO, leading to a change in the fluorescenceZnO), as shown Fig. 3, curve a, a main peak at 590 nm and center of ZnO. So the Kq here can not be called the ‘‘bimolecular quenching

constant.’’a new shoulder at about 640 nm was found in the absorption

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290 CHEN, NICKEL, AND REN

the particle surface, it is calculated that for 1 1 1003 MZnO sol containing 7 mM NTC, every NTC molecule shouldoccupy an area of 400 A2 . This value is much larger thanthe actual molecular area of NTC. It suggests that, despitethe aggregation of NTC, it may quench some fluorophoreson the ZnO surface (because the aggregation of NTC wasinduced by its adsorption on the ZnO surface) . Largeamounts of fluorophores are only accessible to NTC if NTCis in solution. Since the aggregation of NTC results in adecrease of NTC concentration in solution, the probabilityof dynamic quenching between ZnO and NTC decreased.

The Stern–Volmer plots using a quencher of NTC dis-solved in 95% ethanol are shown in Fig. 2B. When [NTC]õ 16 mM , a dynamic quenching between ZnO and NTCwas also observed. However, the Kq value was smaller thanthat with a quencher of NTC dissolved in ethanol (Table1). At higher NTC concentration (ú16 mM) , a curvatureof the Stern–Volmer plot toward the Y axis was observed.From the absorption spectrum shown in Fig. 3, curve c, noaggregation of NTC occurred even when the concentrationof NTC reached 28.36 mM in the presence of ZnO. Appar-

FIG. 3. Absorption (a, b, c) and fluorescence (a*, b*, c*) spectra of ently water molecules (whole content in mixed solutionNTC-ZnO mixed solution. (a, a *) 27.8 mM NTC(A) / 8.8 1 1004 M ZnO;

about 0.5%) play an important role in preventing the NTC(b, b *) supernatant liquid of solution a; (c, c *) 28.36 mM NTC(B) / 9.2dye from aggregation. Water is a highly polar solvent (ET1 1004 M ZnO. Excitation wavelength is 580 nm.(30) Å 63.1 kcal/mol) , it is more polar than ethanol (ET

(30) Å 51.9 kcal/mol) . When H2O is present in ethanoladsorbing in a flat orientation with respect to the semicon-ductor surface (24). Another hypsochromically shifted bandis called the Ha band, which is also caused by the adsorptionof dye on the semiconductor. More importantly, the aggrega-tion of NTC dye itself was initiated by these adsorptionsupon standing for several minutes, flocculation occurred inthe solution and deep purple precipitates were observed. Theabsorption spectrum of the supernatant liquid (Fig. 3, curveb) has a shape similar to that of curve a in Fig. 3, suggestingthat almost all the NTC monomer was transferred into theaggregation state. After the aggregation, little fluorescenceof NTC was observed (Fig. 3 curves a* and b*) .

Figure 4 shows the difference absorption spectra (thespectra of NTC solution in the presence of ZnO minus thatof equimolar NTC solution alone) with increasing NTC con-centration at 207C. A rapid increase in the absorbance peakat 640 nm, corresponding to the shoulder in Fig. 3, curve a,was observed. The appearance of the isosbestic point at 622nm suggests that there exists an equilibrium between theNTC monomer state (602 nm) and the aggregation state (640nm). At about 7 mM , a sudden increase in the absorbance at640 nm (Fig. 4, insert) and a decrease in the absorbance at602 nm (Fig. 5) were observed concurrently. It becomes

FIG. 4. Difference absorption spectra of equimolar NTC(A) with 1clear that the sudden decreased quenching ability of NTC1 1003 M ZnO (sample compartment) minus that without ZnO (referenceshown by the Stern–Volmer plots in Fig. 2A is caused bycompartment) . NTC(A) concentrations increased in the direction of the

the sudden change of the monomer states of NTC into aggre- arrow are 2.48, 3.69, 4.9, 6.1, 7.28, 8.45, 9.62, and 10.77 mM . The insertgation states. Assuming that a ZnO particle with diameter shows the dependence of absorbance difference at 640 nm on the concentra-

tion of NTC(A).of 3 nm contains 1000 ZnO units and NTC can evenly cover

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291QUENCHING OF ZnO BY DYE: WATER EFFECT

FIG. 5. Absorbance difference at 602 nm with increasing NTC concentration.

solution, it would tightly combine with ionized NTC mole- [NTC(C)] Å 3.69 mM (Fig. 6, curve b), the solution wastransparent and NTC was in the monomeric state. As thecules. It is possible that H2O molecules act as a kind of bridge

between NTC molecules, preventing the mutual contact and concentration of NTC(C) increased to 7.28 mM (Fig. 6,curve c) , the increased background absorbance at 650 nm,the aggregation of NTC monomers. In this case, the fluores-

cence of NTC is still strong (Fig. 3, curve c*) . along with a red shift of the absorption edge of ZnO, indi-cated that coagulation of ZnO occurred. This implied thatThe absorbance difference at 602 nm of NTC(B) –ZnO

mixed solution is shown in Fig. 5. No sharp changes were water molecules (content about 2.9%) were adsorbed on thesurface of ZnO particles and destroyed its double chargeobserved as the NTC concentration increased. This supports

the conclusion that no aggregation states were formed duringthis process.

In the case where the NTC quencher was dissolved inwater, owing to the relatively high water content in the mixedsolution (1.5%–7%), ill-defined Stern–Volmer curves andtemperature dependence were observed (Fig. 2C). However,when [NTC(C)] ú 7 mM the quenching efficiency ofNTC(C) was greatly increased, leading to a strongerquenching efficiency than that under the other two conditions(Figs. 2A and 2B). The aggregation states of the NTC atthis concentration also changed. Tracing the difference ab-sorption spectra with continuous increase in the concentra-tion of NTC(C), a new peak located at 518 nm appearedat [NTC(C)] Å 7 mM . At the same time, the absorbanceat 602 nm, which indicated the monomeric state of NTC,decreased (Fig. 5) . Apparently, the enhanced quenchingability was caused by the change in the aggregation stateof NTC.

The presence of water results in not only the change inthe aggregation states of NTC, but also the stability of the FIG. 6. The absorption spectra of 1 1 1003 M ZnO with increasingZnO sol. Figure 6 shows the absorption spectra of ZnO sol concentration of NTC(C): (a) 0, (b) 3.69, (c) 7.28, (d) 10.77, (e) 14.15,

and (f ) 17.44 mM .with increasing concentration of NTC(C). When

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292 CHEN, NICKEL, AND REN

results in the coagulation of ZnO ultrafine particles, leadingto a great decrease in the surface area of the ZnO particles.Then, this hydrophilic surface easily attracts NTC dye fromsolution and forms a new aggregation state, resulting in astrong quenching.

In the present work, it was shown that the interactionbetween ZnO ultrafine particles and NTC dye can be alteredby the variation of water content in NTC–ZnO mixed solu-tion. This provides new possibilities for the changing of thestructure of dye on the surface of ultrafine particles, and itcould also be very useful for the further study of the electrontransfer properties between dye and ultrafine particles.

ACKNOWLEDGMENTS

Thanks to Max-Planck Gesellschaft for the financial support. S. Chenthanks Ms. K. Moules for the helpful suggestions in the preparation of thismanuscript. This work was partially supported by the National NaturalScience Foundation of China and the Eastman Kodak Company.

REFERENCESFIG. 7. Absorption (a, b) and fluorescence (a *, b *) spectra of 12.5 mM

1. Graetzel, M., Acc. Chem. Res. 14, 376 (1981).NTC(C) in the absence (a, a *) and presence (b, b *) of 5 1 1004 M ZnO;2. Henglein, A., Top. Curr. Chem. 143, 113 (1988).supernatant liquid of solution b after centrifugation (c, c*) ; redispersed3. Dimitrijevic, N. M., and Kamat, P. V., Sol. Energy 44, 83 (1990).centrifugate in supernatant liquid (d, d *) . Excitation wavelength is 580 nm.4. Brus, L., J. Phys. Chem. 90, 2555 (1986).5. Kokers, R. J., Acc. Chem. Res. 6, 226 (1973).6. Filby, W. G., Mintas, M., and Gusten, H., Ber. Bunsenges. Phys.

layer. Increasing the concentration of NTC(C) from 10.77 Chem. 85, 189 (1981).mM to 14.15 mM (Fig. 6, curve d and e), the absorbance at 7. Klier, K., Adv. Catal. 31, 243 (1982).

8. Courty, P., Durand, D., Freund, E., and Sugier, A., J. Mol. Catal. 17,602 nm remained constant but a new band with its peak241 (1982).position at a shorter wavelength appeared; a new aggregation

9. Cao, R., Pan, W. X., and Griffin, G. L., Langmuir 4, 1108 (1988).state of NTC containing water was formed.10. Patrick, B., and Kamat, P. V., J. Phys. Chem. 96, 1423 (1992).

Figure 7 shows the absorption spectra of 12.5 mM 11. Kamat, P. V., and Patrick, B., J. Phys. Chem. 96, 6830 (1992).NTC(C) in the absence and presence of 5 1 1004M ZnO 12. Koch, U., Fojtik, A., Weller, H., and Henglein, A., Chem. Phys. Lett.

122, 507 (1985).(curve a and b), respectively. A new peak at 528 nm instead13. Bahnemann, D. W., Kormann, C., and Hoffmann, M. R., J. Phys.of the 602 nm peak appeared in the latter case and the

Chem. 91, 3789 (1987).corresponding fluorescence intensity decreased (Fig. 7,14. Spanhel, L., Weller, H., and Henglein, A., J. Am. Chem. Soc. 109,

curve b *) . Some reddish violet precipitates were observed 6632 (1987).in the mixed solution after it was left standing for several 15. Spanhel, L., Henglein, A., and Weller, H., J. Phys. Chem. 91, 1359

(1987).minutes. After centrifugation, the absorption spectra of the16. Henglein, A., Kumar, A., Janata, E., and Weller, H., Chem. Phys.supernatant solution is similar to that of 12.5 mM NTC(C)

Lett. 132, 133 (1986).solution but with a decreased overall absorbance (Fig. 7,17. Hasse, M., Weller, H., and Henglein, A., J. Phys. Chem. 92, 482

curve c) . This suggests that NTC exists in the form of the (1988).monomer in solution, whereas an aggregation state was ob- 18. Spanhel, L., and Anderson, M. A., J. Am. Chem. Soc. 113, 2826

(1991).served in the precipitates ( in contrast to the result obtained19. West, W., and Gilman, P. B., in ‘‘The Theory of the Photographicin the case of NTC(A), Fig. 3, where aggregated NTC was

Process, 4th Edition’’ (T. H. James, Ed.) , p. 251. Macmillan Co.,also seen in the solution). If the centrifugate was redispersedNew York, 1977.

in the supernatant solution with sonification, the same spec- 20. Kemnitz, K., Yoshihara, K., and Tani, T., J. Phys. Chem. 94, 3099tra as that before centrifugation was obtained. A dynamic (1990).

21. Yonezawa, Y., Hanawa, R., and Hada, H., J. Imag. Sci. 34, 249equilibrium between NTC monomer and the aggregates on(1990).ZnO surface exists. A similar equilibrium was also observed

22. Chen, S., and Nickel, U., unpublished data.for J aggregates (25).23. West, W., and Greddes, A. L., J. Phys. Chem. 68, 837 (1964).

So, it is clear that the enhanced fluorescence quenching 24. West, W., Carroll, B. H., and Whitcomb, D., J. Photogr. Sci. 1, 145by NTC(C) is caused by the formation of a new aggregation (1953).

25. Herz, A. H., Photogr. Sci. Eng. 18, 323 (1974).state on the surface of ZnO. The introduction of water first

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